Monopole mass spectrometer

ABSTRACT

Improved electrode mounting means for a monopole mass spectrometer comprising a rigid end plate and a flexible end plate and means for rigidly supporting one of the electrodes therebetween and flexibly supporting the other.

United States Patent Robert H. Dillenbeck Altamont, N.Y. 674,579

Oct. 1 l 1967 Oct. 19, 1971 lnventor Appl No. Filed Patented AssigneeGeneral Electric Company MONOPOLE MASS SPECTROMETER 4 Claims, 4 DrawingFigs.

U.S. Cl

Int. Cl

Field of Search 1101339/34 ..250/41.9o, 41.9 SB,41.92

Primary Examiner-William F. Lindquist Attorneys-F rank L. Neuhauser,William S. Wolfe, Gerald R.

Woods, Oscar B. Waddell and Joseph B. Forman ABSTRACT: Improvedelectrode mounting means for a monopole mass spectrometer comprising arigid end plate and a flexible end plate and means for rigidlysupporting one of the electrodes therebetween and flexibly supportingthe other.

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MONOPOLE MASS SPECTROMETER This invention relates to monopole massspectrometers generally and more particularly to a monopole massspectrometer having an improved analyzing tube structure which permitshigh resolution of the mass to be analyzed.

Mass spectrometers are well-known devices which are particularlyapplicable for use as partial pressure analyzers or for different gasdetermination. In general, a mass spectrometer operates to analyze gasions in a precisely determined uniform electrical and/or magnetic fieldwherein, depending on the mass of the ions, the ions describe differenttrajectories. These ions may then be collected at different positionsbased upon their ion trajectories and the current generated thereby is ameasurement of certain gas characteristics which may include the kind ofgas or the density of the gas.

A monopole mass spectrometer or analyzer is to be distinguished from theprior analyzing devices wherein the interaction between electric andmagnetic fields causes separation of the particles and from thequadrupole mass spectrometer or analyzer in which a beam of ions islongitudinally accelerated along and within an array of four parallelrods to which rods balanced electrical potentials are applied. Amonopole mass spectrometer is one in which a single conductingelectrode, usually cylindrical, is partially surrounded by a metallicshield electrode in the form of a V, which V simulates the zeroequipotential lines of the quadrupole. In a monopole partial pressureanalyzer or spectrometer, ions of the gases being analyzed are injectedinto the mass analyzer region of the analyzer tube by conventionalNier-type ion source. After passing through an aperture into the areabetween the electrodes, the entering ions are subjected to a combinedunidirectional and alternating electric field which is applied to theelectrodes, a predetermined set of combined voltages normally beingapplied to the conducting rod with the metallic shield, V-electrode,normally being grounded or at ground potential. Depending upon themagnitude of either or both of the applied voltages, the relativeproportional relationship therebetween, the frequency of the appliedalternating current voltage, and the field radius, the equations ofmotions of charged particles in such a field (Mathieu functions)demonstrates that for any given set of operating parameters, onlyparticles of a very small range of charge-to-mass ratio will have stabletrajectories and pass through the analyzing region. Thus, a particularmass may be chosen out of those directed along the length of themonopole for collection by a collector so that a signal is generated bythe collector which is indicative of the concentration of the selectedmass. By varying any of the aforementioned parameters different massesmay be chosen to be impinged upon the detector. By predetermined andregular varying or cyclic varying of one or more of the aboveparameters, a range of masses may be swept so that an analysis over awide range of masses may be conducted in a predetermined period of time.

Considering the number of variables already present in theabove-referred-to equations which will determine the range of particleswhich provide a signal at the collector, it is readily apparent that foran analyzer to have good resolution no undeterminable variable of theelectric field can be permitted along the length of the analyzer, andthat the positioning and structure of the electrodes must be extremelyaccurate and stable.

Accordingly, it is an object of this invention to provide an improvedmonopole mass spectrometer having an accurate and stable electrodestructure.

Another object of the invention is to provide a mechanical andelectrical interference-free electrical field for a monopole massspectrometer.

Yet another object of the invention is to provide an electric field fora monopole mass spectrometer by a pair of elongated parallelspaced-apart electrodes wherein the spacing of the electrodes isaccurately and uniformly maintained.

A further object of the invention is to provide improved electrodes, fora monopole mass spectrometer which may be readily manufactured andeasily assembled in accurate relation.

In accordance with-the above-stated objects, there is provided amonopole mass spectrometer comprising a pair of Iongitudinally extendingelectrodes defining an analyzing region therebetween, means connectingsaid electrodes to a source of electric potential for establishing anelectric field between said electrodes throughout said analyzing region,and means for insulatively supporting said electrodes in parallel,spaced relation to one another, said last-mentioned means comprising arigid end plate and a relatively flexible end plate, means for fixedlysecuring one of said electrodes to said end plates in electricallyconducting relation thereto and means for flexibly supporting the outerelectrode to said end plates in electrically insulating relationtheretol 7 Further objects and advantages of the invention will beunderstood from the following complete description of the preferredembodiment thereof and from the drawings wherein:

FIG. 1 is a diagrammatic representation of a monopole mass spectrometeror analyzer;

FIG. 2 is a side elevational and partial sectional view of an analyzertube of the preferred embodiment of this invention;

FIG. 3 is a view of the left end of the analyzer tube of FIG. 2; and

FIG. 4 is a view of the other end of the analyzer tube shown in FIG. 2.

A monopole mass spectrometer or analyzer is illustrated in diagrammaticform in FIG. 1 of the drawing and includes an ion source I, an analyzertube 2, and an ion detector 3 mounted within an evacuable enclosurestructure 4. Suitable electric power supplies 5, 6 and 7 are providedand connected to the respective elements of the analyzer throughvacuumtight feed-throughs. The ion source may be conventional and may beconveniently the same ion source as is disclosed in copendingapplication Ser. No. 327,617 filed Dec. 3, I963, now U.S. Pat No.3,230,360 issued Jan. I5, 1966, and assigned to the assignee of thepresent invention. The ion detector means 3 may be the electrodestructure of a Dumont-type 24 l -1 l9 photomultiplier tube having inputleads for supplying power thereto and a pair of output leads for readinginformation therefrom, which leads are connected to power supply 7,which is of conventional design and may include an oscilloscope and/orrecorder output. The power supply 6 for the analyzer tube 2 may be ofthe type disclosed in copending application Ser. No. 646,947 filed June19, 1967, and assigned to the assignee of the present invention..Theenclosure 4 may be of any type which may be evacuated by a suitablepumping system and provided with an inlet for the gas to be analyzed(not shown). If the analyzer were used to detect gases in outer space,such auxiliary equipment and enclosure would be unnecessary.

The monopole mass analyzer tube 2 is also illustrated in diagrammaticform in FIG. 1 of the drawing and basically comprises a pair ofelectrodes 8 and 9, electrode 8 consisting of a single centrally locatedmetallic rod electrode to which an electrical voltage is applied andelectrode 9 being a V-shaped metallic shield electrode partiallyenclosing rod electrode 8.

In the operation of the monopole spectrometer, after evacuation of thestructure 4 and introduction of the gas to be analyzed, ions are emittedinto the space between rod 8 and shield 9 and carefully controlledinterrelated alternating and unidirectional voltages are applied to rodelectrode 8 to establish a uniform predetermined electric fieldtherebetween. Thus, only a single mass-to-charge ratio of ions istransmitted through the analyzer region and is incident upon detector '3at a particular interval. The incident ions provide a signal which isthen amplified and recorded by the power supply and signal output 7.

The preferred embodiment of analyzing tube 2 of the monopole massspectrometer in accordance with the invention is illustrated in FIG. 2wherein radiofrequency rod electrode 8 and V-shaped shield 9 are mountedwithin an envelope member 10, which envelope comprises a tube orcylinder II which may be of stainless steel or like material terminatedat its ends by flanges l2 and 13. The flange 13 should be of suitabledesign to coact with a flange 14 (FIG. 1) on the type of enclosure 4with which the analyzer is to be used. The electrodes 8, 9 are securedto an end plate 15, end plate being secured to the flange 12 by anysuitable securing means which as illustrated includes the shoulders 16made a part of flange 12 which cooperate with the overlying screws 17.

An aperture 18 is provided in the central portion of end plate 15 topermit entry of ions from ion source into the analyzing space between RFelectrode 8 and shield electrode 9. The shield 9 is secured to the endplate 15 by suitable means such as, for example, by tap screws 19. TheRF polepiece rod electrode 8 is insulatively supported from end plate 15by suitable means illustrated in FIG. 2 as comprising a recess 20provided in end plate 15 of suitable configuration so as to provide ashoulder or rest point 21 properly spaced from the electrode 9. Atapered bore or recess 22 is provided in the end of electrode 8 and aceramic sphere or ball 23 insulatively supports and aligns electrode 8with electrode 9. A spring end plate 24 is secured to the other end ofelectrode 9 by suitable means such as the screws 25 illustrated,circular openings 26 (FIG. 4) and 27 being provided therein. The opening26 provides means whereby the ions which maintain their trajectorythroughout the longitudinal length of the analyzer region may exit to beimpinged upon the electron multiplier detector 3 to provide a signalindicating the mass range measured.

The opening 27 provides a seat for ball 28 which is received in atapered bore 29 in the end of the pole electrode 8. It will thus be seenthat the RF electrode 8 may be closely and reproducibly aligned with theshield electrode 9 by maintenance of close tolerance on the location ofthe shoulder 21 and opening 27 with respect to the location of theV-block electrode 9 on end member 15 and 24 respectively.

The electric field established between pole electrode 8 and shieldelectrode 9 by application of energy from power supply 6 must becompletely uniform. Such uniformity, of course, will be affected byirregular or variable spacing between the pole pieces, which iseliminated by the above construction. Another factor to be considered inthe mounting of the pole pieces is that they must be electricallyinsulated from each other which is accomplished by the ceramic balls 23and 28.

Further, the ceramic pieces have been eliminated from the area of theelectric field by recessing the ceramic pieces into the end plates 15and 24 and the electrode 8 as shown. Thus, any accumulated charge on theceramic pieces does not affect the electric field through which the ionsfrom the ion source traverse in proceeding through the analyzer regionof the spectrometer. Thus, the variable that might have been introducedby placement of the ceramic pieces in a region whereby such charge couldaffect the field is eliminated. To further isolate the ceramic piecesfrom the analyzing region, the rod electrode may have a toroidal endportion 30 of reduced diameter so as to more completely surround ceramicball 23.

The rod electrode 8 to which the combined unidirectional and alternatingelectric energy is applied must be a nonmagnetic material, capable ofwithstanding the high temperatures utilized during bakeout of the systemand corrosion resistant to withstand attack from the various gases thatmay be analyzed by the spectrometer. It must also be dimensionallystable and of high strength. While metal-coated high-temperature ceramicsuch as the aluminas or forsterite, or quartz may be used, the stainlesssteels having these characteristics are preferred. A specific example ofsuch a stainless steel is I-Iastalloy alloy-X. It will be appreciatedthat if the rod 8 is of a conductively coated insulating material theballs 23 and 28 need not be of insulating material.

From the above construction, it can be readily seen that the ionanalyzer tube portion of the mass spectrometer may be assembled as aunit upon end plate 15 for insertion within enclo-, sure 10. Toeliminate any bending of the rod pieces due to their own weight whilesupported by end plate 15, a plurality of setscrews 44 are providedwhich generally align the analyzer tube electrodes 8, 9 within thecenter of enclosure 10 and support the spring plate end thereof.

While the above construction is sufficiently stable shockproof for mostpurposes and conditions, for some purposes for which the analyzer may beused it may be desirable to more permanently affix the shield electrode9 to the end plates 15 and 24. Thus, end plate 15 may be provided withwelding recesses 31 so that the shield electrode 9 may be tackwelded tothe end plate 15 at its intersections 32 with the recesses asillustrated in FIG. 3. Similarly the shield electrode 9 may betackwelded to end plate 24 at points 33,-the intersections of thecircular end plate 24 and the circular portion of shield electrode 9(FIG. 4).

The V-Shaped shield electrode 9 preferably is manufactured from a singlepiece of electrically conductive rod of suitable high-temperature,high-strength, and nonmagnetic material. The material must also bedimensionally stable and corrosion resistant. Examples of such materialsare the stainless steels, a specific example thereof being Hastalloyalloy-X. To produce the desired electrode shape, a round stock rod ofsuch material is first planned to give it a chordal surface. A V is thenmilled or broached therein by use of either a tool or a 45 tool used andtwo passes made to cut the the inclined sides of the V. After milling,if necessary, the inclined sides of the V may be further polished butsuch is not generally required if suitable care is taken in operation ofthe milling machine. Such an electrode may be readily manufactured andis dimensionally extremely stable.

The cylinder 11 of the analyzer tube is provided with a plurality ofapertures or slots 34 (FIG. 2) around its periphery at its end adjacentthe flange 13. A toroidal support 35 is secured to the flange l3 and thecylinder 11 is secured within the support 35 by a plurality of lockscrews 36. Suitable means 37 may also be provided on the flange 13 formounting of the ion detector 3 and also appropriate vacuumtight electriclead throughs may be provided thereon.

As before mentioned, the flange 13 is adapted to coact with whatevertype of apparatus in which the monopole mass spectrometer is to be used.As illustrated, the flange 13 is provided with a seal seat 38 and aplurality of apertures 39 adjacent its outer periphery, and flange 14 ofenclosure 4 has mating seal seat 40 and apertures 41. Suitable securingmeans 42 extend through the apertures 39, 41 to secure the flangestogether and clamp upon a sealing ring 43, which sealing ring isnormally of copper to make a vacuumtight seal.

While this invention has been described by way of the preferredembodiment thereof, it is to be understood that numerous changes can bemade therein by those skilled in the art without departing from theinvention and it is intended that the appended claims include suchequivalent variations as come within the true spirit and scope of theinvention.

What is new and desired to be secured by Letters Patent of the UnitedStates is:

'1. In a monopole mass spectrometer including first and secondelectrodes defining an analyzing region therebetween, means forproviding ions in said analyzing region, means for detecting ionsexiting said analyzing region and means connecting said electrodes to asource of electric potential for establishing an axially uniformelectric field between said electrodes throughout said analyzing region,the improvement of means for insulatively supporting said electrodes inparallel, spaced relation to one another, said means comprising a rigidend plate and a flexible end plate, means for fixedly and rigidlysecuring said first electrode to said end plates in electricallyconducting relation thereto and rigid means coacting with each of saidend plates for supporting said second electrode between said end platesin accurately spaced, parallel relation to said first electrode and inelectrically insulating relation thereto, said rigid means forsupporting said second electrode including a pair of spherical members,said second electrode having tapered recesses in the ends thereofreceiving said spherical members and said end plates being provided withmeans cooperating with said spherical members to secure said secondelectrode therebetween.

2. A monopole mass spectrometer as recited in claim 1 wherein saidsecond electrode is a rod of conductive material and said sphericalmembers are spheres of insulative material.

wherein said second electrode is formed of an insulating material havinga conductive coating formed thereon.

1. In a monopole mass spectrometer including first and second electrodesdefining an analyzing region therebetween, means for providing ions insaid analyzing region, means for detecting ions exiting said analyzingregion and means connecting said electrodes to a source of electricpotential for establishing an axially uniform electric field betweensaid electrodes throughout said analyzing region, the improvement ofmeans for insulatively supporting said electrodes in parallel, spacedrelation to one another, said means comprising a rigid end plate and aflexible end plate, means for fixedly and rigidly securing said firstelectrode to said end plates in electrically conducting relation theretoand rigid means coacting with each of said end plates for supportingsaid second electrode between said end plates in accurately spaced,parallel relation to said first electrode and in electrically insulatingrelation thereto, said rigid means for supporting said second electrodeincluding a pair of spherical members, said second electrode havingtapered recesses in the ends thereof receiving said spherical membersand said end plates being provided with means cooperating with saidspherical members to secure said second electrode therebetween.
 2. Amonopole mass spectrometer as recited in claim 1 wherein said secondelectrode is a rod of conductive material and said spherical members arespheres of insulative material.
 3. A monopole mass spectrometer asrecited in claim 2 wherein said electrodes are of stainless steel.
 4. Amonopole mass spectrometer as recited in claim 1 wherein said secondelectrode is formed of an insulating material having a conductivecoating formed thereon.